This invention relates to a resistor utilized in a semiconductor integrated circuit, and more particularly, to an ion implanted resistor which is stable with respect to aging.
In most cases, diffused resistors have been used as resistors in semiconductor integrated circuits, but the recent tendency is to use resistors implanted with ions for the purpose of obtaining high sheet resistance of a small area and having a resistance of more than 1 to 10 kiloohms for the particular area. FIG. 1 shows a cross-sectional view of a prior art ion implanted resistor comprising an N type semiconductor substrate 1, a high resistance layer 2 formed by ion implantation, P.sup.+ regions 3 and 4 for providing non-rectifying contacts, i.e. ohmic contacts, combined electrode and metal wiring layers 5 and 6 for the resistance layer 2, a bias applying electrode 7 for applying a reverse bias to a PN junction formed between the N type semiconductor substrate 1 and the resistance layer 2, an N.sup.+ region 8 for forming an ohmic contact, and an insulating silicon dioxide (SiO.sub.2) film 9.
When the ion implanted resistor shown in FIG. 1 is used in a semiconductor integrated circuit having a breakdown strength of 50 to 200 volts, for example, the N type semiconductor substrate 1 is required to have an impurity concentration of the order of about 1.5.times.10.sup.15 -1.5.times.10.sup.16 ions/cm.sup.3. In order to form an ion implanted high resistance layer 2 having a sheet resistance of about 1-10 K.OMEGA. for a particular area in the substrate, the resistance layer is required to have an impurity concentration of the order of about 5.times.10.sup.16 -5.times.10.sup.17 ions/cm.sup.3. These impurity concentrations are relatively low so that when the high resistance layer 2 with a low concentration is formed on such a low concentration N type semiconductor substrate 1, negative ions existing in the SiO.sub.2 film 9 or in a sealing film, not shown, coated on the surface of the low resistance layer create a negative charge in the SiO.sub.2 film 9, whereby a P type inversion layer will be formed on the surface of the N type semiconductor substrate 1. This phenomenon is enhanced with the passage of time by a positive high voltage applied to the bias electrode 7. When the P type inversion layer is formed on the surface of the semiconductor substrate, the width of the resistance layer increases, thus tending to decrease the resistance value. In an ion implanted resistor, the degree of reduction of the resistance value is very large. FIG. 5 shows the variation in the resistance value caused by aging. The data depicted were obtained by forming 50 ion implanted resistors each having an initial resistance value of 100 K.OMEGA. on a semiconductor substrate and then encapsulating the assembly with an epoxy type transfer molding resin. When an accelerated test was made at an ambient temperature of T.sub.a =125.degree. C. and with a bias voltage of V.sub.B =100 V, it was found that the initial resistance value decreased greatly, i.e. by 75% after 100 hours of the acceleration test. Thus, the prior art ion implanted resistance is not suitable for practical use. For the purpose of obviating such variation in the characteristic, even when a stabilizing film comprising a passivating silicon dioxide film or a double layered passivating silicon dioxide film doped or not doped with phosphor is applied onto the semiconductor substrate by a chemical vapor deposition (CVD) process, a characteristic similar to that shown in FIG. 5 was obtained showing that a stable ion implanted resistor is difficult to obtain.